Generally, in wireless communications systems knowledge of downlink (DL) channel information may be essential for efficient beamforming to one or more users. The transmission of information to two or more users is commonly referred to as spatial multiplexing.
DL channel information may be provided by a receiver (e.g., a mobile station, a user, a terminal, a User Equipment, and so on) to a transmitter (e.g., a base station, a NodeB, an enhanced NodeB, a base terminal station, a relay station, and so forth) over a feedback channel. In frequency division duplexing (FDD) communications systems, the receiver may estimate or measure the DL channel and then feed the DL channel information back to the transmitter. The DL channel information may be feedback in its raw form, a quantized version (a codeword from a codebook known by both the receiver and the transmitter), an index to the quantized version (e.g., an index to the codeword from the codebook), or so on.
In time-division duplexing (TDD) communications systems, when calibrated antenna arrays are used, uplink (UL) and DL channels may be almost identical. Channel reciprocity may be a commonly used term to describe this phenomenon. Since the UL and DL channels may be almost identical, it may be possible for a receiver to transmit a sounding reference signal in an UL channel to a transmitter, the transmitter may measure the UL channel using the sounding reference signal, and the transmitter may use the information about the UL channel in its DL transmission to the receiver.
However, in practical communications systems, the receiver may not have an equal number of receive radio frequency (RF) chains and transmit RF chains. For example, a receiver may have two receive antennas but only one transmit antenna, i.e., a first of the two receive antennas may also transmit, but a second of the two receive antennas may only receive. Therefore, only a portion of the channel state information is available through sounding reference signal measurement, providing incomplete channel state information (I-CSI).